Various electronic elements using semiconductors have recently progressed and the industrial field thereof has been inspected to steadily progress hereafter. As the semiconductor which is a basic material of the industrial field, silicon has been mainly used recently but at present, a compound semiconductor (e.g., GaAs) excellent in light emitting characteristics and high-speed property is steadily progressed.
Various kinds of electronic elements having a desired performance can be generally obtained by applying crystal layers having necessary characteristics to a single crystal substrate by various methods, such as an ion injection method, a diffusion method, an epitaxial growing method, etc. In these methods, the epitaxial growing method has been widely used since the method can accurately control not only the amount of impurities but also the composition and layer thickness of the crystal layer in a wide range.
Known examples of the epitaxial growing method include a liquid phase method, a vapor phase method, and a molecular beam epitaxial method (hereinafter referred to as "MBE method") which is one of vacuum vapor deposition methods. In particular, since the vapor phase method can process a large amount of substrates with a good controlling property, the vapor phase method has been widely used in an industrial scale. Among the vapor phase methods, a metal-organic chemical vapor deposition method (hereinafter referred to as "MOCVD method") has recently been widely used.
A high electron mobility transistor (hereinafter sometimes referred to as "HEMT", which is also sometimes called modulation doped field effect transistor (MODFET) or hetero-junction field effect transistor (HJFRT), which is important as a part for constituting a low-noise amplifier for a microwave communication, is one of electric field effect transistors. The crystals used in the electric field effect transistors can be prepared by growing laminated layers of a GaAs crystal and an AlGaAs crystal having necessary electronic characteristics on a GaAs substrate to have necessary structures by the vapor phase method as described above.
As the materials used for preparing these elements, GaAs series materials and AlGaAs series materials have been widely used, since the crystal constants can coincide with each other in optional compositions and various kinds of heterojunctions are possible for them while keeping the good crystalline properties.
Since In.sub.y Ga.sub.(1-y) As (0&lt;y.ltoreq.1) is excellent in the electron transporting characteristics and can greatly change the energy gap according to the composition, it has excellent properties as a hetero-junction material. However, since the lattice conformity to GaAs is impossible, an InP substrate to which the lattice conformity is possible at the value of y of about 0.49 has hitherto been mainly used.
With the recent progress of techniques in this field, it has been clarified that even in a lattice unconformity system, a reliable hetero-junction is possible without causing the undesirable deterioration of crystalline property, such as the occurrence of dislocation, etc., if the crystals are within the limit of the elastic deformation by controlling the composition and layer thickness.
By utilizing a strained layer within such specific ranges of composition and layer thickness, even in an epitaxial substrate using the GaAs substrate, a substrate having the InGaAs layer at a part of the epitaxial substrate can be produced. For example, under the ordinary crystal growing condition, an In.sub.y GA.sub.(1-y) As layer of y=0.15 and of about 15 nm in layer thickness can be grown on the GaAs substrate without deterioration of the crystalline property. By employing the epitaxial substrate having such a structure that the In.sub.y Ga.sub.(1-y) As layer is inserted between a GaAs buffer layer and an n-type AlGaAs electron donating layer, an HEMT excellent in noise characteristics as compared with conventional ones can be produced.
The epitaxial substrate using GaAs as the substrate and containing the strained layer of InGaAs at a part thereof has been produced by the MBE method or the MOCVD method as described above. However, there are problems in the properties and the productivity of the element using the epitaxial substrate. That is, the MBE method is an epitaxial growing method excellent in the thin layer controlling property, but the crystal formed by the MBE method has many surface defects, thereby there is a problem in the yield of the elements produced and also there is a problem in the productivity since the crystal growing speed is slow and the MBE method requires a super-high vacuum.
The MOCVD method is excellent in the productivity and the epitaxial substrate obtained by the MOCVD method has less surface defects. However, there is a problem in that the characteristics of the element using the resulting epitaxial substrate are not always satisfactory. In the case of using the epitaxial substrate prepared under an ordinary crystal growing condition by the MOCVD method, for example, in the case of an In.sub.0.15 Ga.sub.0.85 As layer, the layer thickness deviates from 15 nm (the desired value) to have unevenness of about 2 to 5 nm at a period of from 200 to 400 nm. The inventors have found a problem in that the mobility of the two dimensional electron gas traveling in the InGaAs layer is lowered by the influence of the unevenness.